There is a need for a simple, inexpensive high rate liquid/solid separation apparatus and method to abate pollution from agriculture and urban point and non-point sources. A large number of liquid/solid separation techniques are used in the wastewater treatment industry. In the selection of a suitable separation apparatus and method for a given application, the surface-loading rate of the system is often important to the design. The surface-loading rate is generally reported in gallons per square foot per day (gal/ft2/day) for dilute flows or in pounds of dry solids per square foot per day (lb/ft2/day) for concentrated flows (suspended solids >0.1%). Hydraulic loads are important to the design of the separator when, for instance, turbulence inhibits the necessary separating action. If solids removal is important, then solids loading should be the design criteria. Most conventional flotation separators are desired to have high solids surface loading capacity, yet are unable to achieve this for reasons which will be explained below. A separation process is also selected based on its ability to remove a wide variety of pollutants such as oil and grease, bacteria, colloidal, and suspended solids.
Separation using principles of buoyancy (i.e., flotation separation) is advantageous because it achieves high capture rates while producing a clean effluent. Flotation separation can also concentrate the waste (or recycle) solids. Concentrated waste streams are desirable to minimize the size of downstream processing facilities. Flotation separation has also been successfully used for the efficient removal of suspended solids, colloids, oil and grease (O&G), nutrients, bacteria, organic acids, algae, cryptosporidium, etc.
Conventional flotation separation, however, is considered a complex process, involving gas saturation and injection accompanied by both surface and bottom solids removal apparatus. Recently, the development of an efficient, yet simple, saturator pump has reduced the complexity of the process. However, where the influent waste stream results in a low solids loading rate (<50 lb/ft2/day), the vessel needed to carry out acceptable flotation separation tends to be excessively large.
The maximum hydraulic loading design rate for typical flotation separators is about 5,760 gal/ft2/day (4 gpm/ft2). In actual practice, the flotation separation process may, however, be limited by its solids surface loading rate if the solids concentration of the influent stream is high. This is especially true if the process is used for thickening as opposed to clarification. Thickening refers to concentrating solids to a smaller volume, where clarification generally refers to removing solids. The maximum solids loading rate for conventional flotation separation methods is about 50 lb/ft2/day. Thus, according to FIG. 1, at the maximum design hydraulic loading, the allowable concentration of solids of a typical flotation separator is about 0.1%. However, in actuality, the solids concentration may be much higher, and consequently the typical flotation separator is operating below its maximum hydraulic load. Thus, in almost all circumstances, typical flotation separators are designed and operated to achieve their maximum solids surface load capacity of about 50 lb/ft2/day.
Maximizing the float solids concentration is advantageous since the solids concentration will determine downstream processing resources and cost. If the solids produced are dilute, the downstream dewatering or disposal costs will increase. If the separator is used in a biological process incorporating solids recycle, the processing cost and reactor size will be much greater if dilute solids are produced. FIG. 2 shows how the processing costs increase as the separator's concentration efficiency decreases.
It is desirable to improve the existing flotation separators because the advantages of flotation separation as a method for concentrating waste streams are numerous. Flotation separation can be used for both clarification and thickening. Flotation separation can remove suspended solids, colloids, and oil and grease at the same time. If reagents are added to the flotation stream, nutrients can be removed and consolidated with the solids. If polymers are used, bacteria and a variety of other organisms will be removed. If air is used, the effluent liquid will be aerated. If gas is used, a variety of physical and chemical processes can be implemented. Flotation takes advantage of the hydrophobic interactions that are lacking in other separation technologies.
One attempt to improve flotation separation is proposed in U.S. Pat. No. 6,126,815, to Kelada. Kelada discloses a zero pool velocity flotation separation process and separator vessel. The vessel according to Kelada has a single nozzle for receiving the waste fluid and solids, and for discharging the solids float blanket. In other words, Kelada charges the separator vessel with an amount of liquid waste containing solids. The initial charge is allowed to consolidate for a set period during which no other streams are introduced into the vessel or removed from the vessel. During the consolidation period, the solids rise to the surface and form a blanket of solids. Depending on the amount of consolidation time, the density of the blanket of solids can vary. However, since waste liquids are shut off after one tank volume is charged into the separator vessel, the maximum amount of solids that can be removed is predetermined and cannot exceed that which was initially charged into the separator vessel. As such, the surface loading (lb/ft2/days) capabilities achieved by this apparatus are low.
It is desirable to produce a flotation separator apparatus and method capable of increasing the solids surface loading capabilities beyond what is presently accepted as the maximum. Such an apparatus would have a smaller footprint than conventional flotation separator vessels, thus making it highly economical. The apparatus disclosed herein fulfills such needs.